Illumination is used to light objects for seeing, as also for photography, microscopy, scientific purposes, entertainment productions (including theatre, television and movies), projection of images and as backlights of displays.
For illumination purposes, the present art has many systems in the form of point or single dimensional sources of light. Such systems have many drawbacks: light intensity is very high at the light source compared to the rest of the room or environment, and thus such light sources are hurtful to the eye. Such sources also cast very sharp shadows of objects, which are not pleasing to the eye, and may not be preferred for applications such as photography and entertainment production. Such sources also cause glare on surfaces such as table tops, television front panels and monitor front panels.
There are prior systems that act as light sources in the form of a surface. Fluorescent lights for home lighting may be covered by diffuser panels to reduce the glare. These systems are bulky. They are also not transparent. Diffusers and diffuse reflectors such as umbrella reflectors are used as light sources for photography and cinematography, but they are only approximations to uniform lighting.
Backlights of flat-panel screens such as LCD screens provide uniform or almost uniform light. Prior solutions for backlighting an LCD screen is to have a light guide in the form of a sheet, with some shapes such as dots or prisms printed on it to extract light. The light guide is formed by sandwiching a high refractive index material between two low refractive index materials. The shape and frequency of dots is managed such that uniform illumination over the surface is achieved. Uniformly illuminating a single-dimensional light source, not a surface. These methods give uniform illumination over the surface, but the illumination is not uniform locally—when looked at closely the appearance is that of dots of glowing light surrounded by darkness. Such non-uniformity is not pleasing to the eye, and will cause disturbing Moiré patterns if used as a backlight for a flat panel screen. Such systems, to achieve local uniformity of light, need to be covered by diffuser panels or film, which makes them costlier, bulkier and non-transparent.
There are systems which provide uniform illumination over a surface in the local sense, i.e. locally, a surface is uniformly illuminated. These systems are similar to the systems described above, in the sense that they use a light guide and a method of extracting part of the light being guided. The light extraction, though, is not done with dots or geometric shapes, but with microscopic light scattering, diffracting or diffusing particles. Such particles are distributed uniformly throughout the light guide. This causes a continuously lighted light source, rather than one that is discretely lighted.
On the other hand, as the light is guided from one end of the sheet to another, part of the light is extracted, causing lesser and lesser light left for extracting, and thus lesser and lesser illumination. Thus, these systems do not provide uniformity of illumination over the entire surface. To provide approximate uniformity, the total drop in light from one end of the light guide to the other should not be too large. This, though, will cause light to be wasted at the edge of the light guide, and thus the energy efficiency of the system goes down.